Congestive heart failure (CHF) is a clinical syndrome in which an abnormality of cardiac function causes cardiac output to fall below a level adequate to meet the metabolic demand of peripheral tissues. CHF can be due to a variety of etiologies with that due to ischemic heart disease being the most common. Some form of cardiac pacing can often benefit CHF patients. For example, sinus node dysfunction resulting in bradycardia can contribute to heart failure which can be corrected with conventional bradycardia pacing. Also, some CHF patients suffer from some degree of AV block such that their cardiac output is improved by synchronizing atrial and ventricular contractions with dual-chamber pacing using a programmed AV delay time (i.e., atrial triggered ventricular pacing or AV sequential pacing).
A common sequela of CHF is dilation of the heart chambers (especially the left ventricle) as end-diastolic volume is increased in the body's attempt to increase stroke volume. The ventricles can then become stretched and less contractile which actually worsens the heart failure. Stretching of the ventricular wall can also cause slowed conduction of depolarization impulses through the ventricle. If conduction velocity is slowed in the left ventricle more than the right, for example, the contraction of the two ventricles during ventricular systole becomes uncoordinated which lessens pumping efficiency. Some CHF patients also suffer from conduction defects of the specialized conduction system of the heart (a.k.a. bundle branch blocks) so that a depolarization impulse from the AV node reaches one ventricle before the other. In both of these situations, cardiac output can be increased by improving the synchronization of right and left ventricular contractions. Cardiac pacemakers have therefore been developed which provide pacing to both ventricles. (See, e.g., U.S. Pat. No. 4,928,688, issued to Mower and hereby incorporated by reference.)
Due to stretching of the atrial walls caused by the cardiac dilation described above, CHF patients are predisposed to occurrence of atrial tachyarrhythmias. Atrial tachyarrhythmias are cardiac rhythms characterized by atrial contractions occurring at a rapid rate, either due to an ectopic excitatory focus or abnormal excitation by normal pacemaker tissue. Atrial tachyarrhythmias can be classified according to increasing rate into entities that include atrial tachycardia, atrial flutter, and atrial fibrillation. Due to the refractory period of the AV node, some degree of AV block is usually always present so that the ventricular rate is less than the atrial rate if the AV conduction pathway is otherwise intact. In atrial fibrillation, the atria depolarize in a chaotic fashion with no effective pumping action, and the ventricles beat both rapidly and irregularly due to conduction of excitatory impulses from the fibrillating atria through the AV node. In atrial tachyarrhythmias, and especially atrial fibrillation, the atria no longer act as effective primer pumps for the ventricles which decreases stroke volume, referred to as a loss of atrio-ventricular synchrony. Also, when the ventricles contract at irregular intervals, the contraction can occur prematurely before diastolic filling is complete and decrease the stroke volume for that contraction. An episode of atrial tachyarrhythmia can thus depress cardiac output and cause such symptoms as dyspnea, fatigue, vertigo, and angina. This is especially problematic in CHF patients who are already hemodynamically compromised.